In the petroleum exploration and production industries, downhole tractors can be used to convey tools and other devices into boreholes. However, downhole tractors may be used for any desired purpose. As used throughout this disclosure, the terms “tractor”, “downhole tractor” and variations thereof refer to a powered device of any form, configuration and components capable of crawling or moving within a borehole. The term “borehole” and variations thereof refer to and include any underground hole, passageway, or area.
Traditionally, the movement of the tractor is enabled by friction-generated traction between one or more components associated with the tractor, referred to herein as the “traction module(s),” and a wall of the borehole. In such instances, a normal force is usually applied by the traction module against the borehole wall. For a tractor to achieve or maintain movement within a borehole, the traction module or portions thereof preferably do not slip relative to the borehole wall. The traction module can provide sufficient traction to transport itself, any conveyed tool, and/or a delivery cable into a borehole.
Any number of other factors (referred to throughout this patent as “disturbance factors”) may affect the amount of traction necessary to move the tractor within the borehole in any particular situation and environment of operation. For example, when the borehole wall possesses an irregular surface, the amount of traction necessary for movement and/or the coefficient of friction may change as the borehole surface navigated by the tractor changes. Other disturbance factors that may affect the tractor's resistance to motion include changes in the inclination of the borehole, diameter of the borehole, surface of the borehole, borehole wall properties, increasing cable drag (when a cable is used), debris in the borehole and borehole fluid properties.